It is known that lymphocytes, which appear as a unique population of cells in the peripheral blood of animals, including man, contain two functionally different sub-populations of cells known as T cells and B cells. In view of indications that the relative concentrations of the B and T cells may serve as indicators of certain physiological or pathological conditions in the body of the animal, many attempts have been made to develop a method for obtaining differential counts of the T and B cells in a blood sample. These attempts have been complicated by the fact that B and T cells cannot be distinguished on the basis of their morphology.
As present, the most widely used methods for enumerating the sub-populations of T cells and B cells in blood samples are based on immunofluorescence and rosette formation with erythrocytes. These methods require the preparation of purified suspensions of lymphocytes by methods which are laborious and which usually cause a loss in a particular sub-population of cells. In addition, the morphology of the cells counted as T or as B cells can be determined only with difficulty on stained preparations. Further, these methods permit counting only one type of cell at a time, i.e., either T cells or B cells, but not both simultaneously.
More recent methods for simultaneous identification of T and B cells in leucocyte suspensions use fluorescent antibody of two different colors, e.g., fluorescein and rhodamine, but the use of these procedures as routine tests is precluded by the difficulty, the special equipment, and the extensive training of the operator which they entail.
The use of bacteria to identify some lymphocytes in suspension is known. For example, staphylococcus aureus, strain Cowan I is known to bind to cells bearing IgG, a class of immunoglobulins present on only some B cells. The use of this bacteria does not therefore afford a means of distinguishing between B and T lymphocytes